1. Field of the Invention
The present invention relates to an optical disc apparatus for recording and/or reproducing information on/from an information surface of a rotatable optical disc.
2. Description of the Related Art
An optical disc apparatus of the kind indicated above is known from U.S. Pat. No. 6,052,357. This patent describes an optical disc apparatus for recording and/or reproducing information on/from an information surface of a rotatable optical disc, comprising a supporting assembly and a spindle motor, associated with the supporting assembly, having a spindle with a spindle axis for rotating the optical disc, mounted on the spindle, about the spindle axis. An optical lens unit is associated with the supporting assembly in order to scan an information surface of the optical disc mounted on the spindle. The optical lens unit comprises a focussing lens assembly having a movable focussing lens having a focussing lens area bounded by a focussing lens periphery and a focussing axis intersecting the focussing lens area, the focussing lens assembly being adapted to focus an optical beam on the information surface of the optical disc. A swing arm assembly is provided which comprises a generally elongate swing arm structure, the focussing lens assembly being mounted at a free end thereof. The swing arm assembly is rotatable about a swing axis spaced from the free end and directed generally perpendicular to the swing arm structure and generally parallel to the spindle axis and the focussing axis, such that the swing arm assembly sweeps a plane generally parallel to the information surface of the mounted optical disc, the swing arm assembly thereby causing the focussing lens assembly to scan over the information surface of the mounted optical disc. The swing arm assembly also comprises movable magnetic focussing means, near the free end of the swing arm, associated with the swing arm assembly for driving the focussing lens along the focussing axis to focus the optical beam on the optical disc information surface.
In order to focus the optical beam on the optical disc information surface, this known optical disc apparatus also comprises stationary magnetic focussing means, associated with the supporting assembly, for magnetically cooperating, through an intermediate air gap, with the movable focussing means in order to generate a magnetic force vector having a vector component parallel to the focussing axis of the focussing lens assembly so as to drive the focussing lens assembly along the focussing axis by electromagnetic force.
In this prior art optical disc apparatus, the movable magnetic focussing means comprises magnetic focussing coil means disposed in a plane which is generally parallel to the focussing axis of the focussing lens at the free end of the swing arm structure, at a location spaced from the focussing lens assembly at the remotest part of the swing arm structure as seen from the swing axis thereof. Focussing movements of the focussing lens assembly in a direction according to its focussing axis are enabled by the fact that the swing arm structure comprises two spaced elongate, generally parallel flat resilient arm elements extending between the swing axis and the focussing lens assembly, that is, interconnected at or near the free end of the swing arm assembly. The stationary magnetic focussing means cooperate with the above-mentioned magnetic coil means over an air gap which is generally parallel to the magnetic focussing coil means and, therefore, generally parallel to the focussing axis of the focussing lens.
Any magnetic force vector acting through the air gap on the focussing coil of the movable magnetic focussing means of the prior art arrangement described will exert on the focussing lens not only a force along the focussing axis, but also a perpendicular force. Moreover, these forces act on the focussing lens assembly and, therefore, on the swing arm assembly at some distance from the focussing axis of the focussing lens, resulting in force moments exerted on the resilient arm elements of the swing arm assembly. Thus, eccentric forces and moments are dynamically produced during the focussing of the focussing lens; this will produce dynamic reaction forces and moments in the swing arm assembly which may cause undue vibrations and distortions as well as added loading of the swing arm spindle. Undesirable parasitic resonance may result in the direction along the focussing axis, upsetting the automatic focussing control system of the optical lens unit. Also crosstalk may arise in the scanning direction perpendicular to the focussing direction, upsetting the automatic scanning control system.